As known, in many medical applications, and in particular in the fields of medicine and surgery, filters treated with hygroscopic salts are currently used. The filters are employed as humidifying elements to provide a respiration process which is similar to the normal physiological action of the respiratory tract.
Italian Patent Application No. 21794 A/90, filed on Oct. 19, 1990, discloses a hygroscopic cartridge for filters which is made substantially of a band constituted by first and second layers of paper impregnated with hygroscopic salts. The layers are superimposed and have mutually opposite corrugations forming a plurality of channels which extend transversely with respect to the plane of lay of the bobbin.
This type of filter, which has proved to be valid in some respects, is however not devoid of inconveniences. It is relatively difficult to manufacture since the channels are created by forming corrugations on paper bands and thus, during winding, the channels may be crushed such that the transverse cross sectional passage area of the channels is substantially reduced.
A further disadvantage resides in the fact that, during inhalation, air entering the filter creates a significant drop in the temperature of the filter cartridge, with a consequent increase in condensation and a reduction in the moisture content of the aeriform flow which the patient breathes.